Optical fibres for transmitting high optical power are frequently used in industrial applications. Specifically they are used in cutting and welding operations by means of high-power laser radiation, but also in other industrial applications such as heating, detection or working operations in high-temperature environments this type of optical fibres can be used. By means of the optical fibres it is possible to design flexible manufacturing systems for transmitting the laser beam from the high power laser source to the workpiece. An optical fibre typically has an inner glass core and a surrounding layer, a so-called cladding, having a lower refractive index than the glass core. The function of the cladding is to keep the optical beam to the core. Laser sources which can be used in this context have an average power from a few hundred watts up to several kilowatts.
When designing fibre systems for such high power laser radiation it is important that the fibre is not damaged in any way because the radiation from a “leaking” or damaged fibre might cause serious personal injuries. Therefore it is previously known to check the status of the fibre by means of specific monitoring systems. See for instance U.S. Pat. Nos. 4,812,641, DE 4032967, DE 3246290, DE 3031589 and U.S. Pat. No. 5,497,442.
However, it is important to detect a damage in the optical fibre not only for security reasons but also for preventing secondary damages in other parts of the system due to said damages or imperfections in the fibre.
A weakness in all of the detecting systems referred to above is the fact that the detection of a damage comes too late. When the radiation from the damaged fibre is detected, secondary damages in the optical system might already have occured.